Effect of alterations in frequency, inspiratory time, and airway pressure on gas exchange during high frequency jet ventilation in dogs with normal lungs.

High frequency jet ventilation (HFJV) is becoming increasingly useful for providing respiratory support in patients with normal lungs during operative procedures, and also has been advocated as a technique for ventilating patients during cardiopulmonary resuscitation. We studied the effect of frequency, percent inspiratory time (I/E ratio), peak airway pressure, and airway pressure difference (peak-PEEP) during HFJV as operational variables on the efficacy of gas exchange in dogs with normal lungs. We observed that at a constant peak airway pressure and percent inspiratory time, PaCO2 generally increases as frequency rises above 100/min. In contrast, PaCO2 generally decreases as percent inspiratory time is reduced at a constant frequency and peak airway pressure. In addition, increasing peak airway pressure and airway pressure difference are associated with lower levels of PaCO2. Arterial oxygenation was adversely affected by frequencies above 300/min, but was otherwise not influenced by alterations in frequency, percent inspiratory time, or airway pressure.

A simple way of administering oxygen to the pediatric patient with upper airway obstruction.

An open-top oxygen tent has been developed for supplying humidified oxygen to the pediatric patient while allowing easy access to the child's head, neck, and upper thoracic region with limited impairment of either observations or movement of the child. This tent is easily and economically fabricated from 0.002-in clear plastic sheeting and 3/4-in polyvinyl chloride pipe available from a local hardware store.

High frequency ventilation applications in the OR.

High frequency ventilation provides effective gas exchange at frequencies between 60 and 2,400 min-1. It can be potentially useful in the operating room for bronchoscopy, laryngoscopy, airway surgery, general surgery, and microscopic neurosurgery.

Physiologic consequences of high frequency jet ventilation.

High frequency jet ventilation provides effective gas exchange at frequencies of 60 to 900 minute-1. The selection of mechanical ventilatory support is based on multiple factors, including potential physiologic advantages and disadvantages. Although HFJV has demonstrated no clear significant advantage over CMV in many clinical applications, because it produces significantly lower peak airway pressures than CMV, cardiac compromise and barotrauma might be prevented. In addition, in situations in which tissue movement should be minimized, e.g., microneurosurgery, HFJV may prove beneficial.

Monitoring system gas concentrations.

With proper measurement of inhaled and exhaled gas concentrations as well as end-tidal concentrations, patient concentrations of oxygen, carbon dioxide, and the volatile anesthetic agents may be inferred; additional information for vigilance monitoring, control of depth of anesthesia, and calculation of physiologic variables can be obtained. From this information, the anesthesiologist will be able to administer improved patient care through more immediate and complete knowledge and control of both equipment and patient function.

Why new delivery systems?

Although anesthetists have accomplished a remarkable safety record with commercially available anesthetic machines, these results have been obtained in spite of machine design, which could best be described as a nonsystem. In cases involving severely compromised patients, surgical procedures that severely alter patient physiology, and untoward events during "routine" anesthesia, it is a tribute to the flexibility and resourcefulness of anesthetists that more incidents do not occur. Industry has long sought precision, reliability, automatic control, and human-factors engineering in nonmedical applications, such as aircraft cockpit design, word-processing stations, and manufacturing processes. The relentless accretion of more and more nonintegrated gadgets onto an antiquated technology has exceeded the boundaries of proper function. Neither the patient nor the anesthetist is being served well by failure to implement state-of-the-art technology in anesthesic delivery systems. Anesthesiologists and others who are vitally interested in the welfare of their patients must insist that development of radically new integrated modular systems proceed at full speed. Their checkbooks can speak as loudly as the facts; it is time the manufacturers are aware that deep concern will be translated into purchasing decisions.

Considerations for managing the airway in the ENT patient.

Anesthesiologists may encounter particularly challenging airway management problems and anesthetic requirements for any surgical procedure. However, this is frequently the case with ENT surgery and its patient population. Sharing the airway with the surgeon requires a special understanding of the procedure to be undertaken, clear communication, and cooperation. Basic knowledge concerning the airway anatomy and physiology is essential. Special ENT procedures require special skills. Anesthesiologists may be required to secure an airway in an unconventional manner in these patients, as well as use special ventilation methodology and challenging anesthesia techniques.

Clinical factors influencing the selection of high-frequency jet ventilators.

Criteria for selection of high-frequency ventilators, and in particular high-frequency jet ventilators are not significantly different from those for conventional mechanical ventilators. Selection is based upon the design principles and performance characteristics of the ventilator and successful clinical applications that establish clearly its safety and efficacy. The final choice is also influenced by the physical status of the patient, potential physiologic advantages and disadvantages, the necessary requirements of the clinical situation, and the capability of providing adequate oxygenation and ventilation.

Components of the system: future design requirements.

A few ideas about future design concepts have been outlined and highlighted. Whether anesthetists will accept it or not, automation is the direction of the future, and data processing is increasingly important. Anesthesiology and industry need to initiate total cooperative efforts for appropriate implementation of modern technology.

Investigation of a continuous heating/cooling technique for cardiac output measurement.

Cardiac output is frequently measured to assess patient hemodynamic status in the operating room and intensive care unit. Current research for measuring cardiac output includes continuous sinusoidal heating and synchronous detection of thermal signals. This technique is limited by maximum heating element temperatures and background thermal noise. A continuous heating and cooling technique was investigated in vitro to determine if greater thermal signal magnitudes could be obtained. A fast responding thermistor was employed to measure consecutive ejected temperature plateaus in the thermal signal. A flow bath and mechanical ventricle were used to simulate the cardiovascular system. A thermoelectric module was used to apply heating and cooling energy to the flow stream. Trials encompassing a range of input power, input frequency, and flow rate were conducted. By alternating heating and cooling, thermal signal magnitude can be increased when compared to continuous heating alone. However, the increase was not sufficient to allow for recording in all patients over the expected normal range of cardiac output. Consecutive ejected temperature plateaus were also measured on the thermal signal and ejection fraction calculations were made.

Gas and vapor delivery.

Technology for gas and vapor delivery has not changed substantively in decades. Technology possessing greater precision and reliability has been in use by nonmedical industries to regulate gas flows and to vaporize liquids. Adaptation of existing technology to the needs of anesthesia delivery systems requires stimulus from the anesthesia community and commitment from the anesthesia device industry. No insurmountable problems are evident, but the perennial problem of inertia has prevented progress consistent with that seen in other fields of biomedical technology.

Cardiac output measurements. A review of current techniques and research.

Cardiac output is the volume of blood ejected by the heart per unit time. It is a useful measurement in that it can be used to evaluate overall cardiac status in both critically ill patients and patients with suspected cardiovascular disease. An ideal cardiac output measurement system would have automated continuous output capability, be minimally invasive, accurate, fast, small, low cost and clinically adaptable. This paper presents a theoretical and practical description of the variety of clinical techniques in use today and lists their advantages and shortcomings with respect to the ideal system. Included are the Fick method, indicator dilution techniques, velocity measurements and transthoracic impedance and combined Doppler ultrasound as noninvasive techniques. In addition, several experimental methods are described along with their desirable features and possible constraints. These include intravascular heating/recording, thermistor tracking of cardiac output, ejection fraction measurements and magnetic susceptibility plethysmography.

Effects of dilution, pressure, and apparatus on hemolysis and flow rate in transfusion of packed erythrocytes.

The purpose of this prospective, randomized study was to measure both the independent and interactional effects of dilution, pressure, and apparatus on flow rates and hemolysis during rapid administration of erythrocytes. Administration of undiluted erythrocytes increased the concentration of plasma-free hemoglobin by as much as 270% per unit under certain conditions. Transfusion flow rates for packed red blood cells were found to be determined by dilution, pressure, and apparatus and varied by as much as 450%. No significant correlation was found between flow rate and hemolysis. Regardless of the external bag pressure applied or the transfusion apparatus used, packed erythrocytes should be diluted (with at least 100 ml of normal saline) to decrease hemolysis and increase flow rate.

ECG lead with the endotracheal tube.

We evaluated the potential of obtaining ECG signals from electrodes attached to an endotracheal tube. Tests during surgery using a 2-electrode system with 3 pediatric patients produced recordings suitable for rate monitoring and arrhythmia detection.

High-frequency alternating lung ventilation.

A new mode, high-frequency alternating lung ventilation (HFALV) is described and demonstrated in which the lungs are alternately pulsed. A fluidic oscillator may be used to deliver two pulsed gas streams, 180 degrees out of phase, to bronchial catheters placed via a double lumen endobronchial tube. Inspiratory phase of one lung thus coincides with expiratory phase in the other, and characteristic lateral rocking chest movements is observed. In six dogs, HFALV was compared to simultaneous pulsing of both lungs with comparable flow (18.5 l/min), frequency (144 min-1), and pulse wave shape. Arterial PCO2 was significantly lower (P less than 0.001) with HFALV. Arterial PCO2 was also found to increase linearly (r = -0.862, P less than 0.001) with distance of the catheters' distal tips from the dogs' carinae. Theoretical mechanisms and possible applications of HFALV are discussed.

Airway pressure as a measure of gas exchange during high-frequency jet ventilation.

Airway pressure during high-frequency jet ventilation (HFJV) reflects safety, ventilator performance, and gas exchange. The value of airway pressure as a monitoring and control variable for predicting the effectiveness of gas exchange was examined in 2 studies using healthy dogs. In the first study, HFJV was delivered to the airway via an extra lumen in the wall of an endotracheal tube, at a frequency of 150 cycle/min and 30% inspiratory time. Airway pressures (peak, mean, trough) were measured at various locations, from 5 cm below to 30 cm above the jet port. Pressures measured above the jet were misleading, but the proper measurement distance below the jet remains uncertain. The second study used the same ventilator settings but varied the airway pressure difference between peak and end-expiratory pressures (2, 4, or 6 cm H2O), and either the mean airway pressure (6 or 10 cm H2O) or the positive end-expiratory pressure (0, 5, 10, or 15 cm H2O). The airway pressure difference correlated strongly with efficiency of gas exchange for both CO2 elimination and oxygenation. Mean and end-expiratory pressures showed little influence over moderate ranges, but use of 15 cm H2O of PEEP decreased efficiency of both CO2 elimination and oxygenation, presumably due to increased dead space because of lung overdistension. We conclude that the airway pressure difference, measured as far distal in the airway as is safe and practical, can be useful for monitoring and controlling HFJV.

Serum inorganic fluoride levels in obese patients during and after enflurane anesthesia.

Serum ionic fluoride levels in 24 markedly obese patients (127.6 +/- 6.0 kg) and seven nonobese control subjects (67.3 +/- 1.2 kg) were compared during and following enflurane anesthesia (less than 2.0 MAC hr). Peak serum fluoride levels were higher (28.0 +/- 1.9 vs 17.3 +/- 1.3 micrometers/L, p less than 0.01) and the rate at which fluoride levels increased was more rapid (slope 5.6 vs 2.5 micrometers/L/hr) in obese patients than in control patients. No clinical evidence of nephrotoxicity was found in either group. Vasopressin resistance tests were not performed, and thus it is inknown whether subclinical nephrotoxicity occurred in either study group. Possible reasons for increased enflurane metabolism in obesity are discussed. These possibilities include differences in fluoride ion kinetics, hepatic delivery and penetration of volatile anesthetics, and altered hepatic microsomal enzyme activity. Obesity rather than weight is an important determinant of anesthetic biotransformation.

Response of lower esophageal contractility to changing concentrations of halothane or isoflurane: a multicenter study.

A multiple-center study was performed to determine the relationship between lower esophageal contractility, clinical signs, and anesthetic concentration as expressed by minimum alveolar concentration (MAC). One hundred four American Society of Anesthesiologists Class I through III patients were exposed to isoflurane (with and without nitrous oxide) or halothane in concentrations of 0.5, 1.0, and 1.5 MAC. Heart rate and systolic blood pressure were continuously monitored. Both the amplitude and frequency of spontaneous and provoked lower esophageal contractions were measured in situ by using a 24-F probe equipped with provoking and measuring balloons. Combined results demonstrated statistically significant correlations (P less than 0.001) between lower esophageal contractility and MAC. Spontaneous lower esophageal contractions decreased from 1.10 +/- 0.12 (SEM) contractions per minute (0.5 MAC) to 0.42 +/- 0.05 (1 MAC) to 0.18 +/- 0.05 (1.5 MAC). Provoked lower esophageal contractility values decreased from 45 +/- 4 mm Hg (0.5 MAC) to 29 +/- 3 (1 MAC) to 19 +/- 2 (1.5 MAC). Heart rate changes did not correlate with MAC, and systolic blood pressure correlated in only one of three centers. Intracenter and intercenter analyses failed to demonstrate a significant relationship between lower esophageal contractility and heart rate or systolic blood pressure. No intracenter differences in either amplitude or frequency of lower esophageal contractions were observed, despite differences in volatile agents, induction techniques and agents, patient populations, and durations of anesthesia.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of pulse duration on neuromuscular blockade monitoring: implications for supramaximal stimulation.

Questions have been raised concerning the reliability of surface electrodes in achieving supramaximal stimulation during the monitoring of neuromuscular blockade; needle electrodes are considered reliable in this respect. This study compares interelectrode impedances of needle and surface electrodes during neuromuscular blockade monitoring and suggests those characteristics of the stimulation pulse that can ensure reliable supramaximal stimulation with either type of electrode. Interelectrode voltage and current for surface and needle electrodes were measured by using 1.0-ms pulses at low, medium, and high stimulation levels on 22 surgical patients during anesthesia. Data were collected immediately after electrode application, and again at 10 minutes after application. Stimulation with surface electrodes produced an initial, transient surge of current, followed by a lower steady-state value. At high stimulation levels, the peak transient current was 87% higher than the steady-state current. Needle electrodes produced a constant high-current response. At high stimulation levels the transient impedance of the surface electrode and the impedance of the needle electrode were essentially equal (0.7 k omega and 0.75 k omega, respectively). The transient impedance was significantly lower (P less than 0.001) and was associated with less interpatient variation (P less than 0.001) and less sensitivity to the duration of electrode application than was the steady-state impedance of the surface electrode. These data suggest that high-current pulses with widths of less than 0.2 ms could provide reliable supramaximal stimulation with either type of electrode.

Hemodynamic effects of high-frequency jet ventilation.

The hemodynamic effects of high-frequency jet ventilation (HFJV) and conventional ventilation were compared in normovolemic and functionally hypovolemic dogs. In normovolemic animals, no differences in hemodynamic function were found among spontaneous ventilation, conventional ventilation, and HFJV. When venous return was impaired by 15 cm H2O PEEP, cardiac index and stroke index were 25% higher with HFJV than with conventional ventilation (P less than 0.05). In another study with PEEP, conventional ventilation was compared to spontaneous ventilation, HFJV synchronized to five different parts of the cardiac cycle, and asynchronous HFJV. Heart rate was 15% lower and mean arterial pressure was 26% lower with conventional ventilation than with HFJV modes (P less than 0.05). There were no differences between synchronous and asynchronous HFJV. These results indicate that hemodynamic dysfunction may be less likely with HFJV than conventional ventilation. No advantage of synchronizing jet pulsations to a specific part of the cardiac cycle could be demonstrated.